Stem Volmer plots

The photoreduction of cyclobutanone, cyclopentanone, and cyclohexanone by tri-n-butyl tin hydride was reported by Turro and McDaniel.<83c> Quantum yields for the formation of the corresponding alcohols were 0.01, 0.31, and 0.82, respectively. Although the results for cyclopentanone and cyclohexanone quenching were not clear-cut (deviations from linearity of the Stem-Volmer plots were noted at quencher concentrations >0.6 M), all three ketone photoreductions were quenched by 1,3-pentadiene, again indicating that triplets are involved in the photoreduction. 

The quenching of the trans dimer with oxygen and ferrocene indicates that this product is formed almost entirely from the triplet state. It is possible to calculate the amount of triplet-derived product in benzene by subtracting the amount of product obtained in the presence of oxygen from the amount of product obtained in the absence of oxygen. Such a calculation indicates that acenaphthylene triplets in benzene give both trans and cis dimers in the ratio of 74 26. The triplet state accounts for almost all of the trans product and about 10% of the cis product. The break in the slope of the Stem-Volmer plot for the trans dimer (Figure 10.3) may be attributed to the presence of two excited species which are quenched at different rates. These two species could be (a) two different monomeric acenaphthylene triplet states 7 and T2 or (b) a monomeric acenaphthylene triplet state 7 and a triplet excimer. This second triplet species is of relatively minor importance in the overall reaction since less than 5% of the total product in an unquenched reaction is due to this species. 

Dimers (73) and (74) were formed in approximately equal amounts in all cases, although, as in the cases of 2-cyclopentenone and 2-cyclohexenone, the relative amount of (72) (either cis-syn-cis or cis-anti-cis) was found to vary substantially with solvent polarity. As in 2-cyclopentenone, this increase in the rate of head-to-head dimerization was attributed to stabilization of the increase in dipole moment in going to the transition state leading to (72) in polar solvents. It is thought that the solvent effect in this case is not associated with the state of aggregation since a plot of triplet manifold. However, the rates of formation of head-to-head and head-to-tail dimers do not show the same relationship when sensitized by benzophenone as in the direct photolysis. This effect, when combined with different intercepts for head-to-head and head-to-tail dimerizations quenched by piperylene in the Stem-Volmer plot, indicates that two distinct excited triplet states are involved with differing efficiencies of population. The nature of these two triplets has not been disclosed. 

Figure 10.3. Stem-Volmer plot of the quenching of acenaphthylene dimer formation by ferrocene. (Reproduced with permission from Ref. 41.)...

The commercialization of inexpensive robust LED and laser diode sources down to the uv region (370 nm) and cheaper fast electronics has boosted the application of luminescence lifetime-based sensors, using both the pump-and-probe and phase-sensitive techniques. The latter has found wider application in marketed optosensors since cheaper and more simple acquisition and data processing electronics are required due to the limited bandwidth of the sinusoidal tone(s) used for the luminophore excitation. Advantages of luminescence lifetime sensing also include the linearity of the Stem-Volmer plot, regardless the static or dynamic nature of the quenching mechanism (equation 10) ... 

Figure 1.5 Stem-Volmer plot for a [Ru(dpp)3]2+-doped TFP-TriMOS -propyl Tri-MOS-based sensor film. (Reproduced from ref. 44, with permission.)... 

Fig. 7 Static Stem-Volmer plots. The black line is the ideal Stem-Volmer response, the colored lines illustrate deviations from linearity...

Another issue is how much of a contribution from two sites is required to produce nonlinear Stem-Volmer plots Figure4.14 shows Stem-Volmer plots for another dual distribution data set. r huri = 5, riong = 15, / iong = tfshon = 0.25, and Short = I -0 and k ong = 0.025. However, the fractional contribution of the short-lived component to the total unquenched steady-state luminescence was varied. Clearly, the curvature is pronounced and experimentally detectable from 0.1 to 0.9 not surprisingly, it is more pronounced for comparable contributions from both sites. This last feature is due to the fact that in the limit of pure fast or slow components, the plots become linear. 

When the relation between then nonradiative decay processes and the concentration or value of the parameter of interest k A[Parameter]) is not linear (e.g., Eqs. (9.13 and 9.14), the intensity ratio of Eq. (9.31) introduces the well-observed problem of curvature in the Stem-Volmer plot (see Fig. 9.3). 

Figure 9.3. Stem-Volmer plot, based on luminescence intensity ratios, of heterogeneous sensor-carrier systems. In general, a straight lines are obtained when the dependence of the nonradiative decay rates for all phases are linear (Eq. (9.12)) regardless of the number of phases in the sensor-carrier preparation. Curvature is found when the value of the nonradiative decay rate knr does not increase proportionally with [parameter] (e.g., Eqs. (9.13, 9.14)).

Figure 9.4. Stem-Volmer plot of a fiberoptic oxygen sensor at different temperatures. , 43°C a, 37°C , 31°C , 25°C. The sensing capabilities of the fiberoptic sensor are limited by diffusion processes as suggested by the decreasing value of the lifetimes with increasing temperature. (From Ref. 21 with permission.)...

Whereas a straight Stem-Volmer plot could not be obtained in quenching studies with octafluomaphthalene, the orders of magnitude of the rate constants of triplet decay ka) and of reaction with olefin ( r) could still be estimated Ad = 6 X 10 s i and ki =2 X 10 1 mole is i >). In view of this rapid decay, a high concentration of olefin is evidently necessary for effective addition. 

To verify the quenching interaction between the Re-complex and the di-methyl-/7-toluidine, a Stem-Volmer plot of the results of a concentration dependent study of Re-complex fluorescence intensity as a function of amine concentration in fluid MMA was prepared (Figure 3). The samples contained 1.6 X IQ- mol Re-complex, and up to a maximum of 2.6 X 10 mol of amine, in —2.5 g of MMA. Re-complex CT band peak heights at 612 nm were measured from uncorrected fluorescence spectra taken in single scans following excitation at 350 nm. The Stern-Volmer plot is linear over the range of amine concentrations studied. A linear Stem-Volmer plot. 

Figure 3 Stem-Volmer plot of the results of a concentration dependent study of Re-complex fluorescence intensity as a function of amine concentration in fluid MMA.

Under these conditions, normal Stem-Volmer plots can be expected, both in terms of steady-state singlet-oxygen concentration and decay lifetimes. The slope of the Stem-Volmer plot (Ksy) is given by... 

Figure 32 Sensitization of PBAC fluorescence by NMAC (Top). Stem-Volmer plot for the quenching of NMAC (50 pM) fluorescence intensity by PBAC in the presence of 0.008% BAZrP (ATSV > 1 X 106M 1). (From Ref. 68. Copyright 1994 The American Chemical Society.)...

Figure 34 Stem-Volmer plots for the quenching of fluorescein fluorescence by rhodamine B in (a) water, CTAB, DDAB, CTAB-ZrP, and (b) water, DDAB-ZrP. (From Ref. 69. Copyright 2000 Elsevier Publishers.)...

Figure 1 Stem Volmer plot of fluorescence intensity of the polymer MPS-PPV (inset) in the presence of methyl viologen (MV2+). slope of line gives Ksv = 1.8 x 107 M-1.

In Figure 3.61 we illustrate the difference between the two non-Markovian theories, UT and IET, and their Markovian analog. Inspecting the Stem-Volmer plot, one can see from the insert that the IET curve is nonlinear (quadratic) at low concentrations where the saturation of electron transfer takes place. The Markovian theory in principle reproduces this effect, although less accurately. At higher c both of them reproduce the linear Stern-Volmer dependence r -1... 

Fig. 4.10. Photocurrent spectra with (curve 1) and without (curve 2) 10 3 M hydroquinone (QH2) for Dye II partially aggregated at the surface of WO3 electrode with the use of PD III and the spectral distribution of the relative variation in photocurrent upon insertion of QH2 in the electrolyte (curve 3). The inset shows the Stem-Volmer plots at 560 and 630 nm.

A different explanation was identified in the quenching properties of these heterocycles. Thiophene and mono methyl derivatives are efficient quenchers of triplet benzophenone. The Stem-Volmer plot showed a linear relationship [104, 105]. On the contrary, 2,5-dimethylthiophene (a compound able to give the cycloaddition reaction) is not a good quencher of benzophene [106]. N-Benzoylpyrrole also does not act as a quencher of the triplet benzophenone [106]. On the contrary, pyrrole and selenophene are quenchers of the excited benzophenone [106]. In this case, the Stern-Volmer plot is not linear. This situation is commonly encountered when the quencher employed quenches two excited states. It seems reasonable that pyrrole acts as quencher of both triplet benzophenone and the exciplex between triplet benzophenone and pyrrole [106]. 

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